The computerized tomographic (CT) scanner is a fairly recent development which facilitates examining a body by means of high energy radiation such as X- or gamma-radiation. Using such apparatus one can produce radiographs in any convenient form such as a picture on a cathode ray tube or other image forming device or a photograph of such a picture.
Scanners of this type direct radiation from an external source through the part of the body of interest. The radiation is in the form of a set of beams which are directed from a plurality of different directions toward one or more radiation detectors disposed on the opposite side of the body part from the radiation source. Each beam is detected after it has passed through the body and the outputs of the detectors are processed and correlated by a computer in such a way as to determine the absorption or transmission co-efficients of the elements in a two dimensional matrix of elements disposed in a plane of the body. This processed information is then used to produce a picture of that plane or slice through the body.
In the original type CT scanner made by EMI Limited, the body to be examined is inserted in an aperture in a scanning and locating structure which structure supports a radiation source and detector means therefor on opposite sides of the aperture. The source and detector means are movable back and forth on the structure so as to scan the radiation laterally across the body in a plane and the structure is also rotatable around the body about an axis perpendicular to that plane. A scanner such as that is shown, for example, in U.S. Pat. No. 3,919,552.
The main problem with that prior scanner is that the radiation source and detector arrays and the supporting structure therefor are relatively massive and their movement relative to the body as described above requires motive means and other ancillary equipment which makes the overall apparatus relatively complex, massive and expensive. Also because the body is scanned mechanically, the scanning operation is slow. Resultantly, a relatively long time, i.e. many seconds, is required to complete a scan in order to develop a useful image of a body slice. Consequently, movements of the patient's organs or body during the scan time introduce artifacts into the resultant picture which may obscure parts of the picture and confuse the radiologist. More importantly, however, because they are so slow, such prior scanners are incapable of achieving a rapid sequence of images of dynamic organs such as the beating heart.
More recently, there has been developed a CT scanner having a fixed circular detector array which encircles the patient's body and a radiation source which rotates about the body illuminating a selected plane or slice thereof with a radiation fan beam. The unabsorbed radiation is detected by a different set of detectors for each source position and the information from all of the detectors is correlated to produce an image of that body slice.
While this prior scanner is advantaged in having a fixed detector array, it still requires the mechanical structure to rotate the source in order to produce usable information. Accordingly, it also has a relatively slow scan time, e.g., several seconds for an accuracy of .+-.0.5% or better which is too slow to produce snap shot images of dynamic organs such as the heart. Moreover, it is still relatively complex and expensive because it requires the mechanical structure and motive means necessary to rotate the source. A scanner of that type is made by American Science and Engineering Inc. and is disclosed in that company's publication ASE-3869, dated April 1976 entitled Computerized Tomographic Scanner.
It has also been proposed to construct a scanner in which both the source and the detector array are fixed to avoid some of the aforesaid problems. Two different species have been proposed. In the first, presently under construction at the Mayo Clinic, Rochester, Minn., the source array consists of a fixed array of separate X-ray tubes situated on a semicircle around the patient's body. These tubes are pulsed in sequence to develop a rotating radiation beam which illuminates multiple slices of the patient's body. The emergent radiation is then detected by a fixed semicircular array of detectors diametrically opposed to the radiation source. The signals from the detectors are then processed to develop the picture of the body slice.
While this arrangement has no moving source and detector structure, it is extremely expensive because in order to obtain a reasonably good picture, as many as 28 separate X-ray tubes complete with supporting circuitry and shielding are required. Further, these tubes are relatively bulky so that the different source positions are necessarily spaced relatively far apart. Consequently, the resultant picture does not contain as much information as it should. To compensate for the small number of source positions, the device may have to be rotated slightly during the scan. This requirement, which partially negates the advantages afforded by a stationary scanner, will add to the mechanical complexity and also increase the scan time of the device.
The other proposed species of stationary scanner described in The Journal of Computer Assisted Tomography, Vol. 1, No. 4 dated October 1977 employes a fixed radiation source in the form of an electron beam gun oriented along the patient axis. A ring of X-ray emitting material encircles the patient as does an adjacent fixed circular detector array. The beam from the electron gun is deflected in a circle so that it scans around the target ring causing that ring to emit X-rays radially inward toward the patient. The radiation emerging from the opposite side of the patient is detected by the detector array and the detector signals are processed to develop the picture of the selected slice through the patient's body.
While this type of scanner may have a potential scan time which is fast enough to depict the beating heart in real time, it would be extremely large and bulky because the long (e.g., 3 meters) path of the electron beam from the gun to the target ring must be completely enclosed within a high vacuum chamber (about 10.sup.-7 Torr) to prevent undue electron beam dispersion. Also, a very high current electron beam is required for a fast scan time. Owing to repulsive space charge effects, the beam would be quite difficult to control with the requisite precision (small focal spot size, beam position, etc.) needed to produce a useful picture. For these same reasons, that type of CT scanner would be quite expensive to make and maintain.
None of the presently available CT scanners scan and process information fast enough to produce real time pictures of the beating heart. Rather, they will have to resort to cardiac gating techniques involving averaging the data collected for a given cross-sectional slice of the heart over a series of heart beats. Gating techniques are inherently inaccurate owing to the spatial and temporal non-reproducibility of the heart from beat to beat, and the pictures produced thereby may be degraded by motion artifact blurring.